A gypsum board is a representative gypsum-based building material. Commonly, gypsum board is manufactured according to the following steps. First, foams are preliminarily produced by blowing air into a foaming agent for weight-saving the gypsum board. Then, foams produced from the foaming agent are mixed into a kneaded material that contains a calcined gypsum, an adhesive, various additives and water using a mixer so as to prepare a foamed gypsum slurry to be used as a gypsum core. Then, the prepared foamed gypsum slurry is poured into a space between upper and lower base papers for gypsum board and covered with the base papers for gypsum board. Then, the gypsum slurry covered with the base papers for gypsum board passes through a shaping machine for determining the thickness and width of a gypsum board so as to be shaped into a predetermined board shape. Subsequently, the strip-type original gypsum board that is shaped into a board shape is roughly cut off and passes through a force-drying machine. Finally, the original board after the drying is cut off to a predetermined dimension for a gypsum board product. That is, the gypsum board is a board-shaped structure, in which a gypsum core obtained by the above-mentioned pouring and shaping method is covered with base papers for gypsum board, and has excellent fire resistance, sound insulation, workability and economical efficiency.
In recent years, gypsum board has been used as an interior material in high-rise or super high-rise buildings that are spreading rapidly, as well as in general buildings and low-rise or middle-rise buildings. Gypsum board is known to have excellent properties in regard to adaptability to a building process, weight saving for a building, and flexibility against building oscillation. The weight saving for a gypsum board primarily depends on the ratio of gypsum being a gypsum core material and pores formed by foam, and the less the amount of the gypsum is, that is, the more the amount of the pores formed by the foams, the weight saving is further advanced. However, a reduction in the amount of gypsum may cause the strength of the gypsum core to lower and failure of the adhesion of the gypsum core with the base paper for gypsum board so as to degrade the commercial value of the gypsum board product. Thus, the amount of gypsum is consequentially decided and the weight saving for the gypsum board is limited.
Methods for manufacturing a gypsum board have been studied such that the gypsum board is weight-saved while the strength of the gypsum board is maintained by changing the structure and distribution condition of pores in the gypsum core. As an example of such studies, conventionally, foams obtained by blowing air into a foaming agent exhibiting excellent stability are mixed into a gypsum slurry so as to produce many small pores in the gypsum core, thereby achieving the weight saving for the gypsum board. However, recently, a modification technique for a gypsum core has been proposed which can achieve the weight saving for the gypsum board by containing relatively large independent pores uniformly in the gypsum core.
Such a modification technique for a gypsum core is disclosed in Japanese Patent No. 3028123, in which foams with a desired density are prepared using an aqueous solution of a foaming agent stock solution that contains a particular alkyl ether sulfate and the foams are mixed with gypsum slurry so as to distribute independent large pores uniformly in a gypsum core. Also, a technique whereby foams are produced from a mixture obtained by mixing a particular alkyl ether sulfate as the first foaming agent for forming stable foams in gypsum slurry and a particular alkyl sulfate as the second foaming agent for forming unstable foams in the gypsum slurry with the mixing ratio in a desired range and the mixture of the foaming agent is stirred and mixed with the gypsum slurry, so as to contain relatively large independent pores in a gypsum core, is disclosed in U.S. Pat. No. 5,643,510. Additionally, a technique whereby containing fine pores in a gypsum foamed core is suppressed as much as possible and pores formed by relatively large independent foams are dispersed uniformly by containing a foam stabilizer as well as foams in gypsum slurry for forming the gypsum core is disclosed in Japanese Laid-Open Patent Application No. 10-330174.
In Japan, a gypsum board has conventionally been manufactured using a calcined gypsum obtained by compounding a variety of chemical gypsums such as phosphogypsum, flue gas desulfurized gypsum, neutralized gypsum, and waste recycle gypsum (a gypsum that can be recycled and obtained by scrapping waste materials such as gypsum building materials and gypsum models) and natural gypsum and by calcining a gypsum material that contains the gypsum raw material. However, the sizes and the distribution condition of the pores distributed in a gypsum core are significantly influenced by the kinds and compounding ratios of the above-mentioned gypsum raw materials. In a production plant for gypsum board, the proper gypsum raw materials to use in the plant cannot be freely selected from the various gypsum raw materials and use has to be made according to the fields and demand-supply balance of the respective gypsum raw materials. Consequently, the kinds and compounding ratios of the gypsum raw materials for the gypsum core directly influence the quality of the gypsum board product and have been a particularly serious problem.
Another problem is that the compounding ratios of the gypsum raw materials vary due to various factors so as to effect a bad influence on the pore sizes in the gypsum core in sequential processes from the receipt of the above-mentioned various gypsum raw materials to the manufacture of the gypsum board in the production plant for gypsum board.
In the process of preparing foamed gypsum slurry and the process of shaping a gypsum core, it is difficult to detect the influence of the variations in the kinds and compounding ratios of used gypsum raw materials timely. It is also difficult to form independent pores with a desired size steadily and stably in the gypsum core of the gypsum board product, due to the variations in the kinds and compounding ratios of the gypsum raw materials. Sometimes, innumerable communicated fine pores may be formed in a cross section of the gypsum core, so that the strength of the gypsum core is lowered. An extremely large independent pore may be formed in a cross section of the gypsum core so that the visual appearance of the gypsum board is problematic, and partial peeling of a base paper for gypsum board from the gypsum core, called a cluster or blister, may occur.
For the gypsum core modification techniques disclosed in U.S. Pat. No. 5,643,510 and Japanese Patent No. 3028123, the bad influences of the variety of the gypsum raw materials on the formation of the pores are not discussed and the formation of the gypsum core from a single gypsum raw material is presumed. Accordingly, it is known that if these techniques are applied to a method for manufacturing a gypsum board in which the compounding of a variety of gypsum raw materials is changed, the formation of the pores in the gypsum core is significantly influenced and the sizes and the distribution condition of the pores are widely changed. Also, in particular, in regard to the technique disclosed in U.S. Pat. No. 5,643,510, even if a single gypsum raw material is employed, the average number of moles of ethylene oxide added to an alkyl ether sulfate is only adjusted and it is clear that the range of the controllable sizes of the pores is narrow.
In the gypsum core modification technique disclosed in Japanese Laid-Open Patent Application No. 10-330174, enough uniformly distributed and relatively large pores can be formed in the gypsum core against the variation of the kinds and compounding ratios of the gypsum raw materials but an addition of a relatively large amount of foam stabilizer is needed, so that the production cost for the gypsum board is increased.